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Weed Management in Cotton Guide A-239

Cooperative Extension Service • College of Agricultural, Consumer and Environmental Sciences

INTRODUCTIONCotton (Gossypium spp.; Figure 1) has played an important role in the U.S. economy for more than 200 years. In the colonial period, due to lack of profitability, cotton plantations were mainly concentrated in the eastern U.S. However, as a result of techno-economical ad-vancements, cotton production rapidly expand-ed throughout the southern and southwestern parts of the country, and today cotton is a vital agricultural commodity for many growers in these areas. In New Mexico, the commercial production of cotton started in early 1900, and since 1922 it has become a major socio-economic crop in the state. Despite unsteady trends in cotton prices during the past several years, cotton still remains a major crop in the state; more than 45,000 acres were harvested in 2010 (USDA-NASS, 2011). Among the agro-nomic constraints of cotton production, weed infesta-tions have historically been a major issue. Despite many advances in weed management technology, cotton grow-ers still face significant challenges from weeds.

In cotton, weeds cause several direct and/or indirect negative impacts, such as (a) reducing fiber quality, (b) reducing crop yield, (c) increasing production costs, (d) reducing irrigation efficiency, and (e) serving as hosts and habitats for insect pests, disease-causing pathogens, nematodes, and rodents. Weeds can directly hinder cotton growth by competing for available resources and, in some cases, by releasing allelopathic, or growth-suppressing, chemicals. However, the degree of damage from weed competition is related to the weed species composition (type of weeds), weed densities, and the duration of weed-cotton competition as related to the lifecycle of the cotton plants.

Certain weeds are more competitive with cotton than others mainly because of differences in their growth habit. For example, any weed species that grows taller

than the cotton plants would limit light availability to the cotton, thus causing competition even at low weed densities. In general, as the weed density in cotton fields increases the damage on fiber yield and quality also in-creases. This is due to increased competition between the cotton and weeds for available resources such as light, water, and nutrients. It is important to recognize that the direct negative impact from weeds varies sig-nificantly throughout the lifecycle of cotton. During the early stages of cotton development (i.e., first 8 to 10 weeks after planting, depending on the location), weeds can out-compete cotton seedlings and cause serious damage by reducing the plant vigor. This often results in a reduction in formation of squares and bolls. How-ever, when the crop has become well-established, the cotton plants will be competitive against weeds and the direct negative impact of the weeds on the crop will be minimal. Therefore, for effective weed management in cotton, growers should concentrate their efforts on weed management in the early part of the growing season.

Jamshid Ashigh, Mohsen Mohseni-Moghadam, John Idowu, and Carry Hamilton1

Figure 1. Mature cotton ready for harvest.

1 Respectively, Extension Weed Scientist/Assistant Professor, Department of Extension Plant Sciences; Graduate student, Department of Plant and Environmental Sciences; Extension Agronomist/Assistant Professor, Department of Extension Plant Sciences; and Program Manager, Pesticide Registration, Endangered Species and Pesticide Disposal, New Mexico Department of Agriculture, all of New Mexico State University.

Guide A-239 • Page 2

The minimum time period during which the crop must be kept free of weeds to prevent crop yield loss is referred to as the Critical Period of Weed Control (CPWC). Interference from weeds before or after the CPWC will not result in unacceptable (more than 5%) reductions in yield. However, the CPWC is directly related to weed species composition, weed density, and other crop production practices (fertilization, row spac-ing, etc.), and for this reason CPWC could vary from field to field. More-competitive weed species (e.g., John-songrass, Palmer amaranth) and higher densities should be controlled earlier in the crop cycle than less-competi-tive species (e.g., prickly sida) and lower densities.

INTEGRATED WEED MANAGEMENTThere are five general weed management strategies: pre-ventive, cultural, mechanical, biological, and chemical. Integrated weed management (IWM) requires a system that integrates these management strategies. However, management strategies must be selected based on the field characteristics because the efficacy of each strategy could vary depending on the local environmental condi-tions. In other words, an effective management strategy in one location might not be as effective in other loca-tions. Field characteristics such as soil type, soil pH, water conditions, climatic conditions (hardiness zone), rotational crops, and, most importantly, weed species should be considered when selecting effective weed management strategies. Therefore, IWM uses a combi-nation of effective weed management strategies depend-ing on the environmental conditions. Instead of relying on one particular method of weed control, an IWM system uses a combination of methods. By following the principles of an IWM system, we can reduce the use of herbicides and still obtain optimum economic returns. The use of IWM has been shown to be the most eco-nomical and sustainable way to manage weeds.

Weed identificationThe first step to weed management is to correctly identi-fy and have a record of weeds present in the field. Weeds can be identified using resources such as weed identifica-tion books, websites, and Cooperative Extension Service publications and county agents. In order to make sound management decisions, it is also important to know the biology (life cycle and reproductive mode and capacity) of the present weeds. Keep in mind that some weeds may have several common names, or the same name may refer to different species. Cross referencing weeds by their scientific (Latin) names can help confirm the identity of weeds present in a field. Accurate informa-tion on the biology of the weeds can help growers select effective management techniques. Weeds are often more susceptible to management techniques during specific

stages of their development. Weeds, including grasses, sedges, and broadleaves, are categorized based on their life cycle into annuals (summer annuals and winter an-nuals), biennials, and perennials (herbaceous perennials and simple perennials). Some of the common weeds found in New Mexico agricultural fields and their life-cycles are listed in Table 1.

Weed management strategies i. PreventionThe most important part of integrated weed manage-ment (IWM) is prevention. Growers can prevent weeds from getting into the field by managing weeds in the fencerow or along ditches, controlling weeds before they set seed, planting certified seed, and removing weeds from tillage and harvesting equipment when moving from one field to another. Many of the troublesome weeds, such as field bindweed, Johnsongrass, sandbur, and Palmer amaranth, can be spread from one field to another by harvesting equipment. Preventive manage-ment also requires continuous monitoring of the fields for weed problems. Removing weeds the first time they are noticed prevents them from setting seed and spreading to other areas of the field. For example, small populations of recently introduced weed species can be controlled effectively in the “skip” of skip-row cotton, in row-ends, and in turn-rows. The cost of control increas-es with the size of the weed patch, so it is best to control small weed infestations early and before they become big infestations.

ii. Cultural ControlStudies have demonstrated that cultural crop produc-tion practices (e.g., fertilization, rotation, cover crops, row spacing, and planting date) can influence weed-crop interactions and duration of the CPWC. Cultural weed control emphasizes agronomic practices that benefit the growth of cotton because they create the most favorable environment for cotton, but at the same time the least favorable environment for weeds. Management deci-sions, such as seedbed preparation, time of planting, soil fertility and pH, planting pattern, and moisture, can be manipulated so that cotton is favored. For example, cot-ton planted in a narrow row spacing (25 inches) requires a shorter weed-free maintenance period and becomes more competitive with weed flora than cotton planted with a wider row spacing (38-40 inches). The use of cover crops is also a cultural approach to prevent weed seed germination and increase weed suppression. Crop rotation also has a significant impact on specific weed problems and aids in long-term weed management. For example, inclusion of sudex (sorghum-sudangrass hybrid) either as a cover crop or in a rotation has been shown to suppress weeds in the following crop.

Guide A-239 • Page 3

Table 1. Some of the Common Weeds Associated with Agricultural Fields in New MexicoCommon Name Scientific Name Class

Annuals (plants that complete their life cycle within

one year and reproduce by seed only)

Summer Annuals

Barnyardgrass Echinochloa crus-galli Grass

Junglerice Echinochloa colona Grass

Marestail Conyza canadensis Broadleaf

Pigweed Amaranthus spp. Broadleaf

Sandbur Cenchrus spp. Grass

Sprangletop Leptochloa chinensis Grass

Foxtail Setaria spp. Grass

Kochia Kochia scoparia Broadleaf

Morning glory Ipomoea spp. Broadleaf

Russian thistle Salsola iberica Broadleaf

Southwestern cupgrass Eriochloa acuminata Grass

Lambsquarters Chenopodium album Broadleaf

Winter Annuals

Downy brome Bromus tectorum Grass

London rocket Sisymbrium irio Broadleaf

Shepherdspurse Capsella bursa-pastoris Broadleaf

Flixweed Descurainia sophia Broadleaf

Rescuegrass Bromus catharticus Grass

Tansymustard Descurainia pinnata Broadleaf

Biennials (plants that require two years to complete their life cycle and reproduce by seed only)

Common mallow Malva neglecta Broadleaf

Musk thistle Carduus nutans Broadleaf

Perennials (plants that live more than two years and reproduce by seed

and vegetative reproductive structures, such as root buds, rhizomes, crowns, tubers, stolons, or bulbs)

Simple Perennials

Curled dock Rumex crispus Broadleaf

Dandelion Taraxacum officinale Broadleaf

Creeping Perennials

Bermudagrass Cynodon dactylon Grass

Johnsongrass Sorghum halepense Grass

Silverleaf nightshade Solanum elaeagnifolium Broadleaf

Field bindweed Convolvulus arvensis Broadleaf

Texas blueweed Helianthus ciliaris Broadleaf

Yellow nutsedge Cyperus esculentus Sedge

Purple nutsedge Cyperus rotundus Sedge

iii. Mechanical ControlMechanical weed control is best described as a nonselec-tive control option that is particularly effective against an-nual weeds. Mechanical control is physical weed removal by tools such as hoes, disks, cultivators, rotary weeders, or mechanical choppers. These devices are designed to cover, uproot, or cut weed seedlings. Mechanical weed control starts with the annual pri-mary and secondary tillage practices. Moldboard plowing of the soil leads to the uprooting and shredding of large weeds that have grown in a field during the fall to the early spring season. Moldboard plowing can also bury the weed seeds deep within the soil where they will not be able to emerge. Secondary tillage, such as disking and harrowing, leads to the shredding of the weed biomass and further dislodging of shallow-rooted weeds. Tillage practices are also useful for incorporating some herbicides into the soil to enhance their effectiveness. Although both primary and secondary tillage often lead to a quick de-struction of weeds in a field, they do not provide a lasting solution, especially if weed seeds are still present close to the surface of the soil. Follow up practices using different types of cultivators may be necessary to dislodge and up-root the weeds that emerge after tillage. Mechanical weed control after the planting of cotton is more successful when the weeds are relatively small than when the weeds are large. Therefore, cultivating a cotton field early in the season when the weeds are young will give better results than waiting until later. However, there are some disad-vantages that can result from soil disturbances created by tillage and mechanical weed control implements.

• Tillage tends to create favorable conditions for the germination of weed seeds near the soil surface and bring up new seeds from deeper depths in the soil profile.

• Tillage aids in the dispersal of perennial weeds by breaking up their underground vegetative structures and spreading them throughout the field.

• Frequent tillage could also lead to soil degradation via structure loss, erosion, and compaction.

It is therefore advised to combine mechanical meth-ods with other methods to achieve effective weed con-trol in a cotton field.

iv. Biological ControlOver the past several decades, the concept of biologi-cal control of weeds in cotton has received significant interest. A considerable diversity of biological agents has been used to control weeds, including geese, insects, pathogens, and nematodes. However, these biological methods have shown limited successes in effective control of specific weeds in agronomic crops, particularly cotton.

Guide A-239 • Page 4

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Guide A-239 • Page 6

v. Chemical ControlWeed control in cotton has relied mostly on herbicides, consisting of various functional groups (active ingredi-ents) that are capable of impeding the growth and devel-opment of weeds. A list of currently registered herbicides for cotton in New Mexico and some information regard-ing their usage is given in Table 2. Successful chemical weed control requires the uniform application of the correct quantity of herbicide(s) over the target area. This makes the application of herbicides a precision operation, and accurate calibrations of sprayers are therefore very im-portant since rates that are too high may injure the crop and rates that are too low may not provide weed control. It is also important to use the chemicals at a time when the crop is at its maximum tolerance, and the weeds are at their maximum susceptibility to the herbicides. The sus-ceptibility of both crops and weeds to herbicides is related to the time of application. The following are the terms describing the times at which herbicides may be applied. Unless otherwise specified, they refer to the stage of development of the cotton. All label instructions and precautions must be followed carefully. An herbicide’s poor performance or nonperformance can often be traced to improper use and failure to follow label directions.

Pre-plant. Registered herbicides for pre-plant appli-cation fall into two categories (Table 2):

Pre-plant incorporated (PPI) herbicides, such as tri-fluralin (Treflan) and pendimethalin (Prowl or Acumen), are applied and incorporated into soil 2 to 4 inches deep prior to planting cotton to provide residual weed control. Cotton seed should be planted lower than herbicide in-corporation depth to prevent growth retardation.

Pre-plant burndown herbicides, such as thifensulfu-ron-methyl (Harmony GT XP) and tribenuron methyl (Express with TotalSol), can be applied on emerged weeds prior to planting cotton for a burndown effect. Depending on their chemistry, these herbicides could have residual activity [e.g., flumioxazin (Chateau)] or no residual activity [e.g., glyphosate (Roundup)].

Post-plant. Registered herbicides for post-plant ap-plication fall into three categories (Table 2):

Pre-emergence herbicides are generally applied after planting cotton but prior to weed and cotton emergence for residual weed control. However, some herbicides, such as Diuron (Direx) and pyrithiobac-sodium (Pyri-max 3.2L), have post-emergence activity that can con-trol small seedlings of annual species and also provide residual weed control.

Post-emergence herbicides, such as oxyfluorfen (Ga-ligan 2E or Goal), pyrithiobac-sodium (Pyrimax 3.2L or Staple), sethoxydim (Poast), fluazifop-p-butyl (Fusilade), and clethodim (Select Max or Arrow), are all applied

after cotton emergence. However, some are registered for post-directed application. When cotton plants are 6 inches tall, directed sprays of these herbicides can be used to control small weed seedlings. While herbicides are directed to the row, mechanical control is used for weeds in the furrows.

Lay-by herbicide applications (an application at the final cultivation as the cotton closes over the furrow) prevent the growth of weeds when it is no longer possi-ble to cultivate. Several herbicides are registered for lay-by applications in cotton (Table 2) and they are mainly applied before weed emergence (pre-emergence to weeds), and in some cases are applied to remaining small weeds that were not controlled by previous cultivation and early post-emergence herbicide applications. A lay-by herbicide should be applied as a directed application to minimize contact with cotton plants and can be espe-cially beneficial when cotton plants are short and/or the stand is poor. Without such an application, weeds that escape the previous treatments readily grow and mature with little or no competition from the cotton plant.

Grass control. Certain herbicides, such as se-thoxydim (Poast), fluazifop-p-butyl (Fusilade), quizalo-fop-P-ethyl (Assure II), and clethodim (Select Max), are effective against most grasses, including Johnsongrass and bermudagrass. Select Max is also effective against annual bluegrass. These herbicides are best applied to grasses that are actively growing and less than 6 inches tall. A vigorous and uniform stand of cotton can further enhance Johnsongrass control. Additionally, if the soil is dry, effective control of perennial grasses can also be obtained when rhizomes are cut into short segments by disking and cross-disking during land preparation.

Roundup Ready system. Glyphosate (Roundup) can be applied over-the-top and post-directed to Roundup Ready cotton varieties for the control of annual and pe-rennial weeds. Best control is achieved when weeds are young seedlings. With Roundup Ready cotton, applica-tion timing must be between the cotyledon and 4th true leaf stage. Later applications of Roundup can be made through lay-by (14th node). Roundup can also be used as a spot treatment to control isolated infestations of perennials, or applied with a hooded sprayer to control field bindweed and other persistent weeds. However, Roundup Ready Flex varieties can be sprayed over-the-top with Roundup through lay-by and beyond. For bet-ter weed management, it is recommended that growers consider the CPWC and apply Roundup at early stages rather than late in the season.

Resistance to Roundup in weeds. Due to confirma-tion of resistance to Roundup in some Palmer amaranth populations in New Mexico, it is important that grow-

Guide A-239 • Page 7

Jamshid Ashigh is Extension Weed Specialist and assistant professor in the Department of Extension Plant Sciences at New Mexico State University. He received his B.Sc. in botany and his Ph.D. in weed science from the University of Guelph in Ontario, Canada. His program focuses on integrated weed management systems in field and horticultural crops.

ers not rely only on Roundup for weed management in Roundup Ready cotton. The application of herbicides with other modes of action during the season will not only control Roundup-resistant species (if they exist) but will also prevent or delay the development of resis-tance in existing weed populations.

Liberty Link system. The Liberty Link system uses glufosinate (Rely 280), which has a different mode of action than Roundup, providing broad-spectrum annual weed control with no growth stage restrictions for over-the-top applications. It can be applied to non-Liberty Link cotton with hooded sprayers or post-directed with no rotational restrictions. Because Rely 280 is a contact herbicide, it only provides control of small seedlings of annual species. Field experiments have shown that Rely 280 has better activity on broadleaf weeds than grasses.

Tank-mixing. Tank-mixing herbicides will provide broader weed control; however, it is important to follow the label directions on mixing partners because some herbicides may not be compatible with each other. If such herbicides are mixed, the efficiency of one or both partners in the mixture can be reduced. For example, when Pyrimax is tank-mixed with Poast, Select Max, or Arrow, or if an application of Pyrimax is followed by a Poast or Select Max application, grass control may be delayed. If Pyrimax is tank-mixed with Fusilade or applied within seven days of a Fusilade application, re-duced grass control may occur.

REFERENCESAbernathy, J.R., and C.G. McWhorter. 1992. Evolu-

tion of weed control in cotton. In J.R. Abernathy and C.G. McWhorter (Eds.), Weeds of cotton: Character-ization and control (pp. 1-8). Memphis, TN: The Cotton Foundation.

Buchanan, G.A., 1992. Trends in weed control meth-ods. In J.R. Abernathy and C.G. McWhorter (Eds.), Weeds of cotton: Characterization and control (pp. 47-72). Memphis, TN: The Cotton Foundation.

Mohseni-Moghadam. M., J. Ashigh, J. Schroeder, and R. Heerema. 2010. Glyphosate resistance in a Palmer amaranth (Amaranthus palmeri) population from New Mexico. Meeting of the Western Society of Weed Science, Waikoloa, HI, March 8 to 11. WSWS Abstract.

Supak, J.R., C.G. Anderson, and W.D. Mayfield. 1992. Trends in cotton production: History, culture, mechanization and economics. In J.R. Abernathy and C.G. McWhorter (Eds.), Weeds of cotton: Character-ization and control (pp. 9-45). Memphis, TN: The Cotton Foundation.

Webster, T.M., T.L. Grey, J. Timothy Flanders, and A.S. Culpepper. 2009. Cotton planting date affects the critical period of Benghal dayflower (Commelina benghalensis) control. Weed Science, 57, 81–86.

U.S. Department of Agriculture, National Agricultural Statistics Service. 2011. 2011 upland cotton harvest-ed [Online]. Available from http://www.nass.usda.gov/Charts_and_Maps/graphics/cotnacm.pdf

ACKNOWLEDGMENTSCritical reviews of this article by Dr. Kulbhushan Grover, Dr. Anil Shrestha, and Cooperative Extension Service Agent Jeff Anderson, M.Sc., are acknowledged.

Guide A-239 • Page 8

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